This sounds like the refraction going wild. Also, know that it is important to have something to reflect in the glass. The recent versions of Carrara 7 still have the old presents, so the more advanced options of the shader tree are missing. Look for the shading options like 'reflection' and 'index of refraction' and use options like fresnel effect.

Also, you can look for free shaders and tutorials for this. Glass is hard to get right. There are even expansion packs (shoestringshaders) that add factors that are not in the standard shader tree.